An Orange County drunken-driving trial ended in a mistrial Friday when prosecutors had no witness able to testify about the authenticity of an alcohol breath test given to the defendant.

Prosecutors had attempted to call as a witness a sheriff's official who was disciplined Thursday for falsifying maintenance records on breath-test machines. County Judge Thomas Kirkland ruled the man could not testify because his name was not on a state witness list.

Prosecutors later allowed the defendant to plead guilty to a lesser charge of reckless driving. Friday's case was not related directly to the sheriff's investigation that concluded at least one, and possibly several, breath-test machine logs had been falsified earlier this year by sheriff's employees.

But several defense attorneys said it underscores problems within the unit that operates and maintains the breath-test machines. They said it could affect many of the thousands of drunken-driving cases in Orange County.

Sheriff Lawson Lamar said Friday that he expects many drunken-driving defendants to attack their breath-test results because of the falsified record turned up in the investigation.

''Defense attorneys are always grasping at straws in DUI cases,'' Lamar said. ''The important thing to note here is that we caught it ourselves, and we dealt with it. The system worked.''

Lamar said he has initiated safeguards to make sure maintenance checks are performed and properly recorded.

The investigation was begun earlier this year, after a sheriff's employee reported that her supervisor, Carleton Cook, had recorded a maintenance check on a breath-test machine on Feb. 20, although the check was not done.

Cook told investigators he didn't test the machine that day because he was ''too tired.'' He said he did it two days later.

Cook has been suspended without pay for 10 days and transferred to another assignment.

The maintenance check on the breath machines are important because prosecutors cannot introduce the breath-test results in a trial unless they prove that the checks were made each month.

Prosecutors and defense attorneys gave very different predictions Friday about what effect the sheriff's investigation will have on driving under the influence cases.

Lou Cartwright, supervisor of the Orange-Osceola public defender's misdemeanor division, said the investigation would lead to many challenges of breath-test results in drunken-driving cases.

''The state is supposed to prove in court that the equipment is operating properly and that the required maintenance has been done,'' Cartwright said. ''I don't see how they can do that now.''

Steven Foster, head of the Orange-Osceola state attorney's traffic division, said the case may make it more difficult for prosecutors to get breath-test results into some trials. He said prosecutors won't throw out any cases because of it.

''It means we're going to have to spend a little more time on some of our cases, arguing technical points about evidence,'' Foster said. ''I think it's going to be a minimal matter, not that big a deal.''

Foster said the important issue will be whether jurors believe that sheriff's officials performed the maintenance required to keep the breath-test machines accurate.

Foster and Lamar agree that even in cases where breath-test results are thrown out prosecutors still have other evidence: videotapes of defendants performing sobriety tests and descriptions from arresting officers of how defendants were driving and behaving.

''In many cases, there should be plenty of evidence without a Breathalyzer,'' Lamar said.

Friday morning Orange County Judge Thomas Kirkland declared the mistrial in the case of Jerry Hupp, 38, who was charged with drunken driving.

Kirkland declared a mistrial because no one was able to testify about the authenticity of the alcohol breath test given to Hupp by Cook and a former sheriff's employee, Robert Bickley

Bickley could not be located.

After Kirkland declared a mistrial, prosecutors accepted Hupp's guilty plea on a charge of reckless driving, which they had earlier refused.

Hupp was sentenced to 10 days in jail, a $500 fine and restricted to driving only for business for two years. He has a previous drunken-driving conviction and was facing the loss of his license for 5 years if convicted of drunken driving.

Hupp was arrested April 5, 1984, after the car he was driving was hit by another auto, which careened out of control and hit another vehicle. He registered a 0.19 on the blood-alcohol test, almost twice the legal limit in Florida.